The present invention relates to an image processing apparatus and method for performing, e.g., gradation correction of an input image.
In an image processing apparatus which reproduces an original image on the basis of a digital image signal, when an image is formed at high resolution, in general, graininess (coarseness of the image) becomes conspicuous in a bright (highlight) portion of the reproduced image due to unstable reproduction of individual pixels. Especially, in an electrophotographic digital copying machine as a typical image processing apparatus, as the xcex3 characteristic curve of a xcex3 correction unit has a larger slope, the image is readily influenced by discharging non-uniformity arising from the contaminated charging surface of a charger, which charges a photosensitive drum, resulting in a considerable reproducibility drop of, in particular, a halftone image. For this reason, a scheme for reducing the coarseness of the highlight portion of the reproduced image by performing gradation correction between two neighboring pixels has been proposed.
As an example of the technique for performing gradation correction between two neighboring pixels, assume that the signal levels of a plurality of pixels in a two-dimensional matrix are corrected using a plurality of characteristic curves pre-stored in, e.g., a look-up table (to be abbreviated as an LUT hereinafter).
In such case, a desired characteristic curve shape cannot often be accurately expressed by a digital value stored in the LUT due to quantization errors unique to digital values. For example, when the tangent to a desired characteristic curve has a small slope, data which can be actually stored in the LUT as that characteristic curve represent step-like characteristics locally having steps. After gradation correction is done using such characteristic curve, gradation changes stepwise in accordance with the signal levels corresponding to step portions of the characteristic curve in an output gradation pattern. With this pattern, when an actual original image is reproduced, an unwanted pseudo edge appears in the reproduced image.
It is, therefore, an object of the present invention to provide an image processing apparatus and method having good gradation reproducibility.
In order to achieve the above object, an image processing apparatus of the present invention is characterized by comprising the following arrangement.
That is, there is provided an image processing apparatus comprising correction means for performing gradation correction of odd- and even-line pixels neighboring in a predetermined scanning direction of an image represented by an input image signal in accordance with a plurality of types of correction information having different characteristics,
wherein one of the plurality of types of correction information represents a downward convex correction characteristic curve, and at a given signal level of the input image signal where the correction information that represents the downward convex curve changes stepwise, another correction information changes in correspondence with the stepwise change.
There is also provided an image processing apparatus comprising correction means for performing gradation correction of odd- and even-line pixels neighboring in a predetermined scanning direction of an image represented by an input image signal in accordance with a plurality of types of correction information having different characteristics,
wherein when the plurality of types of correction information respectively represent downward convex correction characteristic curves, and one of the plurality of types of correction information that represent the downward convex curves changes stepwise at a given signal level of the input image signal,
another correction information which is different from the correction information that represents the downward convex curve which changes stepwise is corrected, so that
a difference between a sum of corrected signal levels of the odd- and even-line pixels obtained when the odd- and even-line pixels at a first signal level of the input image signals are corrected by the correction means in accordance with the correction information that represents the downward convex curve which changes stepwise, and a sum of corrected signal levels of the odd- and even-line pixels obtained when the odd- and even-line pixels at a second signal level different a predetermined level from the first signal level of the input image signal are corrected by the correction means in accordance with the correction information that represents the downward convex curve which changes stepwise, assumes a predetermined value.
There is also provided an image processing apparatus for performing correction of an input image signal using a plurality of neighboring pixels as one unit,
wherein in a change portion of a first correction curve applied to one of the plurality of pixels, a change in second correction curve applied to another one of the plurality of pixels corresponds to the change in first correction curve.
Furthermore, there is provided a computer readable storage medium which pre-stores correction information which represents the correction curve having the above-mentioned characteristics.
In order to achieve the above object, an image processing method of the present invention is characterized by comprising the following feature.
That is, there is provided an image processing method for performing gradation correction of odd- and even-line pixels neighboring in a predetermined scanning direction of an image represented by an input image signal in accordance with a plurality of types of correction information having different characteristics,
wherein one of the plurality of types of correction information represents a downward convex correction characteristic curve, and at a given signal level of the input image signal where the correction information that represents the downward convex curve changes stepwise, another correction information changes in correspondence with the stepwise change.
There is also provided an image processing method for performing gradation correction of odd- and even-line pixels neighboring in a predetermined scanning direction of an image represented by an input image signal in accordance with a plurality of types of correction information having different characteristics,
wherein when the plurality of types of correction information respectively represent downward convex correction characteristic curves, and one of the plurality of types of correction information that represent the downward convex curves changes stepwise at a given signal level of the input image signal,
another correction information which is different from the correction information that represents the downward convex curve which changes stepwise is corrected, so that
a difference between a sum of corrected signal levels of the odd- and even-line pixels obtained when the odd- and even-line pixels at a first signal level of the input image signals are corrected in accordance with the correction information that represents the downward convex curve which changes stepwise, and a sum of corrected signal levels of the odd- and even-line pixels obtained when the odd- and even-line pixels at a second signal level different a predetermined level from the first signal level of the input image signal are corrected in accordance with the correction information that represents the downward convex curve which changes stepwise, assumes a predetermined value.
There is also provided an image processing method for performing correction of an input image signal using a plurality of neighboring pixels as one unit,
wherein in a change portion of a first correction curve applied to one of the plurality of pixels, a change in second correction curve applied to another one of the plurality of pixels corresponds to the change in first correction curve.